Offshore Issues Related to Microbiological Fuel Contamination
Fuel oils such as gas oil, diesel and kerosines suffer from microbiological attack (oxidation) due to their chemical composition. They contain a wide range of compounds (diesel contains more than 250) but are primarily composed of n-alkanes (50%, mostly with carbon numbers between 10 and 18), other straight chain, branched and cyclic alkanes, aromatic hydrocarbons (e.g. benzene, toluene, xylene) in variable proportions up to 25% and polycyclic compounds. The fuel therefore has abundant carbon sources for microbiological growth but is deficient in inorganic nutrients such as nitrogen, phosphorous and potassium. These elements are often the limiting factors to microbial degradation of the fuel and must be supplied from and external source, for example the water used in sea water displacement systems, fuel additives or extraneous water entry into the system.
N-alkanes are the most readily biodegraded fraction of the fuel: under oxygenated conditions and with water present complete removal of n-alkanes can take place within a few days. Other compounds may be more toxic or more resistant to attack – for example alkanes with a carbon number less than 9 and some aromatics are highly water soluble and have been shown to resist microbiological degradation. In most cases, however, these compounds can be utilised by one or more microbial groups and will be attacked when physical and/or chemical conditions are favourable. Utilisation of such recalcitrant compounds often occurs after complete biodegradation of the more readily available compounds.
The general chemical formula of the n-alkanes is: CH3(CH2)n-CH3 [simplified to R-CH3] with the carbon atoms arranged in a straight chain. Branching may occur and this generally increased the resistance of the compound to microbiological attack. With straight chain n-alkanes, oxidation occurs by a number of mechanisms which can be represented:
R-CH3 + Reduced Enzyme + O2 —> R-CH3-OH + Oxidised Enzyme + H2O
The process thus requires oxygen and liberates primary alcohols and water. Once microbial oxidation has begun conditions can be self-perpetuating, with free water and further carbon sources being produced. Other microbial groups are now able to utilise the breakdown products, in turn yielding further carbon compounds as below:
R-CH2-OH —> R-CHO —> R-COOH —> CH3COOH Alcohol Aldehyde Fatty Acids Acetic Acid
Once conditions are suitable for microbial growth, therefore, fuel biodegradation may be rapid, with a diverse and very active microbial flora set up in the water phase.